1. Field of the Invention
The present invention relates to an automatic focusing device for a camera which is able to detect a focusing condition of an objective lens of a camera by receiving light beams of an object image having passed through the object lens and to drive the objective lens to a focus position thereof automatically according to data obtained by said detection.
2. Description of the Prior Art
There has been known a focus condition detecting device in which two images are formed by refocusing light bundles of an object image having passed through a first and second areas of an objective lens being symmetric with respect to the optical axis of the objective lens, a relative distance between these two images is calculated and a defocus amount of the focus position detected from a predetermined focus position and a direction of the defocus are determined based on the relative distance calculated.
A typical optical system for such a focus condition detecting device as mentioned above is shown in FIG. 17.
In this system, a condenser lens 6 is arranged on a predetermined focal plane 4 or on a plane positioned behind the focal plane and two refocusing lenses 8 and 10 are arranged rearward of the condenser lens 6. There are arranged two image sensors 12 and 14 on each of focal planes of two refocusing lens. Each of the image sensors 12 and 14 is comprised of a CCD (change coupled device) image sensor having a plurality of light sensing elements.
As shown in FIG. 17 schematically, when an image (A) of an object is focused forward of the predetermined focal plane 4, two images a and a' are refocused on the image sensors 12 and 14 so as to approach to each other with respect to the optical axis 18 of the objective lens. On the contrary to the above, when focused rearward of the predetermined focal plane 4, two images b, b' are refocused apart from each other. If an image is focused just on the predetermined focal plane 4, a distance between two points corresponding to one to one of two images refocused on two image sensors 12 and 14 becomes a specific value which is determined by the composition of the optical system of the focus condition detecting device. Accordingly, a focus condition of the objective lens can be determined from the distance between corresponding two points of the refocused images.
In an automatic focusing device of a camera including the focus condition detecting device as mentioned above, a control circuit including at least one micro computer controls the integration of light by the CCD image sensors to have the CCD image sensors generate image signals corresponding to the intensity distributions of the object images formed on the CCD image sensors by the refocusing lenses, respectively. Thereafter, the control circuit controls calculation of the focus condition (the amount of defocus) based on the image signals from the CCD image sensors, driving the objective lens according to the amount of defocus calculated, stopping the objective lens at a focus position thereof and a shutter release (when the shutter release button is pushed) sequentially according to control programs stored in the micro computer. The automatic focusing device repeats the automatic focus adjusting control towards a focus position successively and due to repeated controls, the objective lens is attained to an exact focus position finally.
Meanwhile, in the automatic focusing device as mentioned above, if an object is moving toward or going away from the camera, an exact focus position can not be attained by moving the objective lens to a focus position according to a defocus amount obtained by one focus condition detecting operation since the object is moving during said operation. FIG. 18 shows a behavior of the automatic focusing device diagrammatically when an object is moving relative to the camera.
In the graph of FIG. 18, horizontal axis represents time and the vertical axis represents an amount of defocus on a film surface.
A continuous curve 1 shows a variation of a defocus amount being taken with respect to the film surface in the case that an object is moving toward the camera and no focus adjustment is carried out. A noncontinuously bent line m is a line obtained by plotting defocus amounts caused when the automatic focusing operation is repeated.
The time t.sub.0 indicates a middle timing during first integration by each of the CCD image sensors. A defocus amount at that time is defined as D.sub.0. During the time from t.sub.0 to t.sub.1, a remaining integration and calculation for obtaining a defocus amount are executed. During the time from t.sub.1 to t.sub.2, the objective lens is driven and then, the objective lens is stopped and the next integration is started during the time from t.sub.2 to t.sub.3. Further, the next calculation is executed during the time from t.sub.3 to t.sub.4.
As shown in FIG. 18, while the defocus cmount increases to the position A due to the movement of the object, the objective lens is driven to move the object image only to a position O. Therefore, a defocus with an amount represented by a difference (D.sub.0 -D.sub.1) results. Next integration is done to obtain a defocus amount (D.sub.2 -D.sub.1) (at the timing t.sub.3), and driving of the objective lens is completed at the timing t.sub.5. However, an image of object by objective lens has been already moved to a position B, and, therefore, a defocus amount (D.sub.3 -D.sub.2) is caused. This defocus amount (D.sub.3 -D.sub.2) is enlarged when compared with the preceding defocus amount (D.sub.2 -D.sub.1).
Similarly to the above, the defocus amount increases as indicated at the positions C (D.sub.5 -D.sub.4) and D (D.sub.7 -D.sub.6) respestively, though the automatic focusing operation is repeatedly carried out. Thus it becomes impossible to attain an exact focus position. Such a delay in the automatic focusing controls as is caused by a moving object becomes more serious in the case that an interchangeable lens having a long focal length such as a telescopic lens is on the camera body mounted.